Conventional emitter-up heterojunction bipolar transistors (HBTs) are known to suffer from diminished performance characteristics because of a large base-collector capacitance. It is well known that the power gain of a bipolar transistor is inversely proportional to the square root of its base-collector capacitance. Collector-up HBTs are advantageous in that the collector area of the transistor can be made significantly smaller than the emitter area. Because of their much smaller base-collector capacitance, collector-up HBTs have considerably higher power gain, higher maximum oscillation frequency (f.sub.max), and faster switching speed than conventional emitter-up HBTs having the same emitter dimensions. These characteristics make collector-up HBTs especially useful for microwave power and digital applications.
Conventional emitter-up and collector-up HBTs are shown in FIGS. 1 and 2, respectively. Comparison of the interface between the base layer 100 and the collector layer 102 in FIG. 1, and the interface between the base layer 200 and collector layer 202 in FIG. 2, illustrates the advantage in base-collector capacitance of the collector-up configuration. Despite this advantage in base-collector capacitance, collector-up transistors suffer from at least one disadvantage. Since the emitter area is larger than the collector area in collector-up HBTs, electrons are injected from the emitter not only into the base region that lies beneath the collector mesa, but also into the so-called extrinsic base region that lies outside the collector mesa, as represented by arrows 204 in FIG. 2. This electron injection into the extrinsic base region results in excess base leakage current and poor emitter injection efficiency.
Two approaches have been taken in the past to overcome the extrinsic base electron injection problem. The first, shown in FIG. 3, forms p-doped regions 300 in the extrinsic regions of the wide bandgap (n-AlGaAs) emitter layer 301. The extrinsic emitter regions lie directly beneath the extrinsic base regions. Since the extrinsic p-AlGaAs/n-AlGaAs junction has a higher turn-on voltage than the intrinsic p+-GaAs/n-AlGaAs junction, the p-n junction formed in the wide bandgap emitter material serves to block electron injection from the n-AlGaAs emitter layer 301 into the extrinsic regions of the base layer 304. The p-doped region 300 can be formed by dopant diffusion, ion implantation, or regrowth. Dopants with fast diffusivities, such as zinc or beryllium, are required in order for the dopants to diffuse through the base 304 to reach the emitter layer 301 underneath. However, the high-diffusivity of these dopants also result in reliability problems. P-type dopant implantation damages both the extrinsic base and emitter layers, resulting in higher base resistance, traps, and non-zero base-emitter leakage current in the implanted regions 300 and the extrinsic portions of the base layer 304 that lie above the regions 300. Extrinsic base and emitter region regrowth requires complicated additional processing steps, and the effects of traps and leakage current along the regrowth boundaries pose reliability problems.
FIG. 4 shows the second and more popular approach to the electron injection problem. Ion implantation is used to convert the wide bandgap emitter layer 400 that lies directly beneath the extrinsic base regions 402 into a highly resistive region 404. Similar to the p-type dopant implantation approach discussed above, this technique results in high base resistance, traps, and non-zero base-emitter leakage current in the implanted regions 404, and the extrinsic portions of the base layer 402 that lie above the regions 404, because of the damage caused by the implantation. The present invention intends to address this and other shortcomings of the prior an approaches.